Redesign gem

Updated all the things (mostly, implemented everything we did on dotcom):

* Renamed gem sir_trevor_rails (used to be sir-trevor-rails)
* Removed sir trevor JS and all its dependencies (should install files manually or from bower from now on)
* Removed all the view helpers appart from sir_trevor_markdown (render_sir_trevor, render_sir_trevor_block, render_sir_trevor_image, sir_trevor_image_tag, parse_sir_trevor are all gone)
* Removed the form helper (no more sir_trevor_text_area)
* Added has_sir_trevor_content.rb as a concern for models with ST content 
* Remove jQuery rails dependency
* Use view resolvers and to_partial_path to render ST content (instead of the view helper)
* Add block array class
* Add block classes
* Add lookup methods on the block array

Take a look at the updated readme file for more info

There are no tests for this yet (:trollface:) but people are having problems with the old gem and this addresses most (all) of those issues.

...so... maybe we should just release it? Unless anyone has time to write the tests soon?